Quarterly Financial Data Net income and earnings per share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Jan. 02, 2021	Sep. 26, 2020	Jun. 27, 2020	Mar. 28, 2020	Dec. 28, 2019	Sep. 28, 2019	Jun. 29, 2019	Mar. 30, 2019	Jan. 02, 2021	Dec. 28, 2019	Dec. 29, 2018
Quarterly Financial Information Disclosure [Abstract]
Net income (loss) attributable to Kellogg Company, Basic	$ 205	$ 348	$ 351	$ 347	$ 145	$ 247	$ 286	$ 282	$ 1,251	$ 960	$ 1,336
Basic	$ 0.60	$ 1.02	$ 1.02	$ 1.01	$ 0.43	$ 0.73	$ 0.84	$ 0.82	$ 3.65	$ 2.81	$ 3.85
Diluted	$ 0.59	$ 1.01	$ 1.02	$ 1.01	$ 0.42	$ 0.72	$ 0.84	$ 0.82	$ 3.63	$ 2.80	$ 3.83